A nonlinear optical element plays an important role in the field of optical communications and optical information processing. A nonlinear optical material for nonlinear optical element is a substance which exerts extremely important effects in the processing of optical signal. For example, it can exert an optical mixing effect to generate a frequency representative of the sum or difference of two incident lights having different frequencies. It also exerts an optical parametric effect to emit light having a frequency different from the original frequency. It further exerts Pockels effect or Kerr effect due to the change in the refractive index of light medium. Further, it can convert incident light into second harmonic (SHG) or third harmonic (THG). Moreover, it exerts a memory effect due to optical bistability.
As the nonlinear optical element material there has heretofore been mainly used an inorganic compound. As such an inorganic nonlinear optical material there has been known an inorganic compound such as potassium titanium phosphate (KTP: KTiOPO.sub.4) and lithium niobate (LN: LiNbO.sub.3) in crystal form. However, one of these inorganic compounds can satisfy the requirements in the foregoing applications.
On the other hand, an organic nonlinear optical material has recently been noted as a new optical element material in the field of optoelectronics and extensively studied. In particular, it has been known that a compound having an electron donative group and an electron attractive group in .pi.-electron conjugated system undergoes an interaction between laser beam as electromagnetic wave and .pi.-electron unevenly distributed in the molecule to exhibit a strong optical nonlinearity at the molecular level (on a molecular basis).
Examples of compounds which have been studied include 2-methyl-4-nitroaniline, m-nitroaniline, N-(4-nitrophenyl)-L-prolinol, 4-dimethylamino-4'-nitrostilbenzene, and 4'-nitrobenzylidene-4-nitroaniline.
Most of these materials are used in the form of single crystal similarly to the inorganic materials. These single crystals need to be noncentrosymmetric to exert secondary optical nonlinearity effect. However, since these materials have a high molecular dipole moment, the noncentrosymmetric crystals can hardly form a thermodynamically stable phase.
Referring to the design of a material which forms an noncentrosymmetric crystal, the introduction of asymmetric centers or the use of hydrogen bonds has been known useful. However, ordinary methods have not been found yet.
Further, problems characteristic of organic materials, i.e., difficulty in crystal growth and fragility of the resulting crystal, make it difficult to precision-process these crystals. It has thus been keenly desired to put into practical use of a high performance material necessary for the preparation of high efficiency elements.
In general, a nonlinear optical element is required to exhibit a high optical nonlinearity, an excellent workability, heat resistance, weathering stability and optical transparency, and a high breakdown voltage and stability upon irradiation with laser beam in combination. However, it is extremely difficult to select from conventional known materials those satisfying these requirements.
The inventors previously proposed cyclobutenedione derivatives represented by the general formula (IV) shown below and nonlinear optical elements comprising these cyclobutenedione derivatives (see JP-A-3-112950 (The term "JP-A" as used herein means an "unexamined published Japanese patent application")). These derivatives are characterized by optical nonlinearity extremely greater than that of known materials. However, these derivatives are disadvantageous in that they exhibit a maximum absorption value (.lambda.max) as great as about 400 nm and thus are not suitable for the generation of second harmonics from a semiconductor laser (oscillation wavelength: 700-800 nm). It has thus been desired to provide a material having a better transparency to visible light. ##STR3##